1. Field of the Invention
The invention relates to silicone-containing polyvinyl acetals, processes for the preparation thereof and the use thereof.
2. Description of the Related Art
Polyvinyl acetals have been known since as long ago as 1924, a large number of aldehydes having subsequently been used for the preparation of the corresponding polyvinyl acetals. Polyvinyl acetals are prepared in a three-stage process (polyvinyl acetate→polyvinyl alcohol→polyvinyl acetal), products which also contain vinyl alcohol units and vinyl acetate units in addition to vinyl acetal groups resulting. In particular, polyvinyl formal, polyvinyl acetacetal and polyvinyl butyral (PVB) have become important commercially. The largest field of use for polyvinyl acetals is the preparation of safety glasses in automotive construction and in architecture, plasticized polyvinyl butyral films being used as an intermediate layer in glass panes. A further field of use for polyvinyl acetals is the use in corrosion-inhibiting coatings. Inter alia because of their good pigment-binding power, polyvinyl acetals are also used as binders in finishes and especially in printing inks.
In said applications, the polyvinyl acetals are modified with silicone in certain cases. In order to improve the resistance of glass laminates having an intermediate polyvinyl acetal layer, JP-A 06-211549 recommends modifying the intermediate layer of plasticized polyvinyl acetal with silicone oil. In U.S. Pat. No. 4,277,299, the addition of polysiloxane resin as a release agent is recommended for glass laminates having a polyvinyl acetal coating. JP-A 60-210551 states that the impact strength of glass laminates having an intermediate polyvinyl butyral layer can be improved by modification with silicone oil. A disadvantage there is that, owing to the poor miscibility of silicones with polymers, the separation of the composition and migration of the silicone fraction may occur.
It is also known from the prior art that polyvinyl acetals can be modified by means of grafting or polymer-analogous reaction with silicone. JP-A 08-73542 discloses a process in which the vinyl alcohol units of a polyvinyl butyral are reacted with methacryloyl isocyanate, and unsaturated trialkoxysilanes are then grafted on. In JP-A 09-141801, polydimethylsiloxane chains are grafted onto the vinyl alcohol units of a polyvinyl acetal. JP-A 02-141289 describes the polymer-analogous reaction of, inter alia, polyvinyl butyral with a polysiloxane. In JP-A 07-290847, polyvinyl acetoacetal is reacted with an OH-terminated polysiloxane in the presence of diisocyanate. The very inconvenient syntheses which are associated with secondary reactions and may lead to mixtures or to crosslinked products are a disadvantage in the preparation of silicone-modified polyvinyl acetals by means of polymer-analogous reaction. The often unstable and frequently insufficient binding of the silicone moiety can moreover lead to phase separation and separation of the product.